I'd imagine it comes down to multitasking - could Dominos successfully deliver pizza at the same time as other items? Each delivery fee would only have to make up the difference of the distance they were already traveling to the additional destination. Considering their coverage, and the fact that the drivers have central hubs and don't have to spend time idling, I think they could offer more competitive pricing while still profiting. They also have more freedom to prioritize pizza/packages as needed, where an Uber driver can't (reasonably) add a stop to their to route with a passenger in car.